Under abuse conditions, high energy density electrochemical cells can leak or rupture which can cause damage to the device employing the cell or to people using the device. Examples of abuse conditions for a cell are abuse charging of the cell, forced discharging and external shorting. Such conditions cause the internal temperature of the cell to rise with a corresponding increase in pressure. Although such cells typically employ a venting mechanism wherein the electrolyte is expelled, the electrolyte can itself cause damage. Additionally, in cells which employ lithium as an anode material, if internal cell temperatures reach above 180.degree. C., the lithium can melt and result in a fire. Therefore, safety devices other than venting means which will disconnect the electrical circuit under abuse conditions are desirable. If the circuit can be disconnected, the rise in the cell's internal temperature as a result of the abuse conditions can be terminated.
One method of interrupting the flow of electrical current in the cell when the internal temperature of the cell rises is to employ a PolySwitch disc, tradename of Raychem Corp., in the cell. Such a disc is a resettable device which, when a threshold temperature is reached, will inhibit the flow of electrical current in the cell. When the temperature is lowered, the disc will allow the normal available current in the electrical circuit to be restored. Unfortunately, these devices are not suitable for all applications. For example, when the PolySwitch disc is designed to function at low temperatures, the disc may have a low breakdown voltage. Once the breakdown voltage is reached, the disc no longer inhibits the current flow. Therefore, if cells containing these discs are used in a multicell device, and abuse conditions are experienced, the breakdown voltage would be reached and the PolySwitch disc would not inhibit the current flow.
In Japanese Patent Publication No. 59-191273, a memory alloy terminal lead is employed in a lithium/thionyl chloride cell. When the internal temperature of the cell rises, the alloy lead wire folds down to break the electrical circuit. When the temperature falls, the lead wire returns to its original shape, restoring the contact. Forming a resettable disconnect member of this single lead wire is undesirable, because the member must be trained to fold down and back again, which requires great expense and time.
In Japanese Patent Publication No. 59-203376 a memory alloy terminal wire is employed in a lithium/thionyl chloride cell to connect the negative or positive electrode to a terminal on the cell housing. The memory alloy terminal wire operates such that as the cell is overheated, the wire will deflect from contact with the terminal and then return to contact the terminal when the temperature within the cell is decreased. The drawbacks of using this type of memory alloy terminal wire are the same as discussed above for Japanese Patent Publication No. 59-191273.
In Japanese Patent 63-72062 a shape memory alloy is placed between a terminal plate and a sealing body of a cell so that the sealing body is broken, thereby venting the cell, when the temperature of the cell exceeds the transformation temperature of the shape memory alloy.
U.S. Pat. No. 4,855,195 discloses electrochemical cells that employ a current collector-safety switch member comprised of a shape memory alloy in the electric circuit in the cell. The collector-safety switch member has a base portion and a plurality of legs extended therefrom and when the internal temperature of the cell rises, the legs of the collector-safety switch member are retracted to thereby disconnect the electrical circuit in the cell. These cells can also be comprised of a resettable thermal switch for inhibiting the flow of current in the cell at a temperature below the disconnect temperature of the current collector-safety switch member.
It is an object of the present invention to provide a safety circuit disconnect device for use in electrochemical cells.
It is another object of the present invention to provide a safety circuit disconnect device that can operate reliably when a cell employing the device is subjected to abuse conditions.
It is another object of the present invention to provide a resettable circuit disconnect device that employs a shape memory alloy twisted wire that is easy to produce and cost effective to assemble in the cell.
The foregoing and additional objects will become fully apparent from the following description and the accompanying drawings.